The present invention concerns calibration of high frequency signal measurement systems. More particularly, but not exclusively, this invention concerns method of calibrating a high frequency signal measurement system and an apparatus for performing such a method. The invention also concerns a method of measuring the response of an electronic device to a high frequency input signal, computer software for controlling a calibration process, and a calibrated network analyser.
The invention relates particularly to the field of RF microwave frequency engineering applications, particularly in applications allowing measurement of absolute phase when analysing, and characterizing, the large signal behaviour of a high frequency device. Such high frequency devices may for example be in the form of high power amplifiers for use in a mobile telephone network. The field of invention also extends to tracking the relative phase between spectral components at different frequencies of signals, particularly RF signals. This includes, for example, tracking between input signals and down-converted output signals of frequency transforming devices such as mixers or multipliers. Such devices are commonly used in receivers and transmitters. To make such measurements a calibrated high frequency measurement system is typically required.
The provision of a calibrated high frequency measurement system that is able to extract not only absolute signal magnitude information, but also absolute signal phase information, from measurements of signals at a measurement plane, may assist greatly with the analysing, and characterising, of the large signal behaviour of a high frequency device, commonly referred to in the art as a device under test (DUT). Such analysis and characterisation is for example useful when seeking to improve or optimize the performance of devices or circuits for use in high power, high frequency amplifiers, such as an amplifier for use in a mobile telephone network.
It will be appreciated that the term “absolute phase” is used herein to refer to the phase of a signal relative to a fixed and known (but perhaps notional) point in time, so that the phase of various signals at different frequencies may be established relative to such a fixed point in time.
Analysis of the behaviour of a DUT is typically carried out with the assistance of a high frequency measurement apparatus, such as a vector network analyser (VNA). A VNA is typically capable of measuring the small signal (linear) performance of a DUT. However, to enable such a measurement apparatus to make non-linear (large signal) measurements, additional calibration steps are required, to ascertain absolute magnitude (power) and phase measurements of signals at the DUT. A VNA, once so calibrated and set-up to make non-linear measurements, is typically termed a non-linear vector network analyser (NVNA). Ascertaining the absolute magnitude of signals from measurements made by the VNA can be achieved by calibrating the VNA with reference to measurements directly made with a power meter. Ascertaining the absolute phase of signals from measurements made by the VNA can be achieved with the use of a phase reference standard, which may be defined by a pre-characterised high frequency signal.
There are proposals in the prior art relating to the provision and use of phase reference standards, such as the paper entitled “Time-Domain Calibrated Measurements of Wideband Multisines Using a Large-Signal Network Analyzer” by Mohammed El Yaagoubi, Guillaume Neveux, Denis Barataud, Tibault Reveyrand, Jean-Michel Nebus, Frans Verbeyst, Francis Gizard, and Jérôme Puech (IEEE Transactions on Microwave Theory and Techniques Vol. 56, No. 5, May 2008, pp 1180 to 1192), which paper proposes a pulse/comb generator made using step recovery diodes (SRD), yielding a multitude of harmonics with a known phase relationship.
An exemplary set-up of a measurement system 102 utilising a phase reference standard system 104 is illustrated in FIG. 1 of the accompanying drawings. The system 104 that generates the phase reference standard (comprising signal components at different predefined and pre-characterised frequencies and absolute phases relative to a trigger signal) is connected to the VNA 106 during any method of measuring the behaviour of a DUT 108. The phase reference standard system 104 uses at least one measurement channel (R5) of the VNA 106 thus reducing the number of measurement channels (R1-R4) available for measuring other signals. Often two measurement channels of the VNA are required to allow the phase reference standard to be properly and fully integrated into the measurement system. A signal generator source (SS2) of the VNA 106 is also required for use solely for the purposes of providing the phase reference standard. It is therefore desirable to provide a method of calibrating a high frequency measurement system, such as a VNA, without requiring the need to have a phase reference standard system connected to the high frequency measurement system during characterisation/measurement of the behaviour of a DUT. Systems for generating phase reference standards can suffer from a lack of reliability/quality (high noise relative to signal levels, for example), particularly at high frequencies, and can therefore be limited in the bandwidth of operation.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method of calibrating a high frequency signal measurement system and/or an improved calibration apparatus for a high frequency signal measurement system.